It is especially in the final week of Advent that our attention is fixed on the Messianic promises proclaimed by the ancient prophets of Israel.  A distinctive feature of the Liturgy of the Hours, in this week preceding the Christmas vigil, is the Antiphon sung at Vespers (the early evening prayer of the Church) before and after the recitation of the Magnificat.  

These Antiphons cover the special period of Advent preparation known as the Octave before Christmas, from December 17th to December 23rd, with December 24th being Christmas Eve and Vespers for that evening being for the Christmas Vigil.

Originally incorporated into the monastic office in the Middle Ages, these Antiphons, often called the "Greater Antiphons" or the "O Antiphons", are also echoed in the daily breviary. They add a mood of eager expectation to the liturgy, that builds throughout these seven days and climaxes at Christmas. 

The exact origin of the “O Antiphons” is not known. The Antiphons date back at least to the reign of Charlemagne (771-814), and the 439 lines of the English poem Christ, by Cynewulf (around 800 AD), are described as a loose translation and elaboration of the Antiphons.  One source stated that Boethius (c. 480-524) made a slight reference to them, thereby suggesting their presence at that time.  Julian reports that two 11th century copies can be found in manuscripts in the British Museum and the Bodleian. The usage of the "O Antiphons" was so prevalent in monasteries that the phrases, "Keep your O" and "The Great O Antiphons" were common parlance.

At the Benedictine abbey of Fleury (now Saint-Benoit-sur-Loire), these Antiphons were recited by the abbot and other abbey leaders in descending rank, and then a gift was given to each member of the community. By the eighth century, they are in use in the liturgical celebrations in Rome. One may thereby conclude that in some fashion the “O Antiphons” have been part of our liturgical tradition since the very early Church. 

According to one source, on December 17th, the Abbot would intone the first Antiphon, O Sapientia. On successive nights, each principal officer of the monastery would take his turn with another of the Antiphons.  After the service, the officer was expected to provide some sort of treat, usually edible, for all the monks.

The O Antiphons have been described as "a unique work of art and a special ornament of the pre-Christmas liturgy, filled with the Spirit of the Word of God".  They "create a poetry that fills the liturgy with its splendor", and their composer shows "a magnificent command of the Bible's wealth of motifs".  The Antiphons are, in fact, a collage of Old Testament types or figures of Christ.  Their predominant theme is Messianic,  stressing the hope of the Savior's coming.  Jesus is invoked by various titles, mainly taken from the prophet Isaias.  The sequence progresses historically, from the beginning, before creation, to the very gates of Bethlehem. 

In their structure, each of the seven Antiphons follows the same pattern, resembling a traditional liturgical prayer.  Each O Antiphon begins with an invocation of the expected Messias, followed by praise of him under one of his particular titles.  Each ends with a petition for God's people, relevant to the title by which he is addressed, and the cry for him to "Come". 

The importance of “O Antiphons” is twofold: Each one highlights a title for the Messiah: O Sapientia (O Wisdom), O Adonai (O Lord), O Radix Jesse (O Root of Jesse), O Clavis David (O Key of David), O Oriens (O Rising Sun), O Rex Gentium (O King of the Nations), and O Emmanuel. Also, each one refers to the prophecy of Isaiah of the coming of the Messiah. 

The Antiphons beg God with mounting impatience to come and save His people. The order of the Antiphons climb climatically through our history of Redemption. 

In the first, O Sapientia, we take a backward flight into the recesses of eternity to address Wisdom, the Word of God. In the second, O Adonai, we have leaped from eternity to the time of Moses and the Law of Moses (about 1400 BC). In the third, O Radix Jesse, we have come to the time when God was preparing the line of David (about 1100 BC). In the fourth, O Clavis David, we have come to the year 1,000 BC. In the fifth, O Oriens we see that the line of David is elevated so that the peoples may look on a rising star in the east, and hence in the sixth, O Rex Gentium, we know that He is king of all the world of man. This brings us to the evening before the vigil, and before coming to the town limits of Bethlehem, we salute Him with the last Great O, O Emmanuel, meaning "God-with-us".

Let’s now look at each Antiphon with just a sample of Isaias’ related prophecies : 
O Sapientia: “O Wisdom, O holy Word of God, You govern all creation with Your strong yet tender care. Come and show Your people the way to salvation.” Isaias had prophesied, “The spirit of the Lord shall rest upon Him: a spirit of wisdom and of understanding, a spirit of counsel and of strength, a spirit of knowledge and fear of the Lord, and His delight shall be the fear of the Lord.” (11:2-3), and “Wonderful is His counsel and great is His wisdom.” (28:29). 

O Adonai: “O sacred Lord of ancient Israel, who showed yourself to Moses in the burning bush, who gave him the holy law on Sinai mountain. Come, stretch out Your mighty hand to set us free.”  Isaias had prophesied, “But He shall judge the poor with justice, and decide aright for the land’s afflicted. He shall strike the ruthless with the rod of His mouth, and with the breath of His lips He shall slay the wicked. Justice shall be the band around His waist, and faithfulness a belt upon His hips.” (11:4-5); and“Indeed the Lord will be there with us, majestic; yes the Lord our judge, the Lord our lawgiver, the Lord our king, He it is who will save us.” (33:22). 

O Radix Jesse: “O Flower of Jesse’s stem, you have been raised up as a sign for all peoples; kings stand silent in Your presence; the nations bow down in worship before You. Come, let nothing keep You from coming to our aid.” Isaias had prophesied, “But a shoot shall sprout from the stump of Jesse, and from his roots a bud shall blossom.” (11:1), and "On that day, the root of Jesse, set up as a signal for the nations, the Gentiles shall seek out, for His dwelling shall be glorious.” (11:10). Remember also that Jesse was the father of King David, and Micheas had prophesied that the Messias would be of the house and lineage of David and be born in David’s city, Bethlehem (Micheas 5:1). 

O Clavis David: “O Key of David, O royal Power of Israel controlling at Your will the gate of Heaven: Come, break down the prison walls of death for those who dwell in darkness and the shadow of death; and lead Your captive people into freedom.”Isaiah had prophesied, "I will place the Key of the House of David on His shoulder; when He opens, no one will shut, when He shuts, no one will open.” (22:22), and “His dominion is vast and forever peaceful, from David’s throne, and over His kingdom, which He confirms and sustains by judgment and justice, both now and forever.” (9:6). 

O Oriens: “O Radiant Dawn, splendor of eternal light, sun of justice: come, shine on those who dwell in darkness and the shadow of death.” Isaias had prophesied, “The people who walked in darkness have seen a great light; upon those who dwelt in the land of gloom a light has shown.” (9:1). 

O Rex Gentium: “O King of all the nations, the only joy of every human heart; O Keystone of the mighty arch of man, come and save the creature You fashioned from the dust.” Isaias had prophesied, “For a child is born to us, a son is given us; upon His shoulder dominion rests. They name Him Wonder-Counselor, God-Hero, Father-Forever, Prince of Peace.” (9:5), and “He shall judge between the nations, and impose terms on many peoples. They shall beat their swords into plowshares and their spears into pruning hooks; one nation shall not raise the sword against another, nor shall they train for war again.” (2:4) . 

O Emmanuel: “O Emmanuel, king and lawgiver, desire of the nations, Savior of all people, come and set us free, Lord our God.”Isaias had prophesied, “The Lord Himself will give you this sign: the Virgin shall be with child, and bear a son, and shall name Him Emmanuel.”  Remember “Emmanuel” means “God is with us.” 

 In Latin the initials of the titles make an acrostic which, when read backwards. means: "Tomorrow I will be there" ("Ero cras").  To the medieval mind this was clearly a reference to the approaching Christmas vigil. According to Professor Robert Greenberg of the San Francisco Conservatory of Music, the Benedictine monks arranged these antiphons with a definite purpose. If you start with the last title and take the first letter of each Antiphon (ignoring the "O").

Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia - the Latin words ero cras are formed, meaning, “Tomorrow, I will be [there].” Therefore, the Lord Jesus, whose coming we have prepared for in Advent and whom we have addressed in these seven Messianic titles, now speaks to us, “Tomorrow, I will be there.” So the “O Antiphons” not only bring intensity to our Advent preparation, but bring it to a joyful conclusion.